Polydiorganosiloxane polymers have unique properties derived mainly from the physical and chemical characteristics of the siloxane bond and the organic substituents. Typically, the outstanding properties of polydiorganosiloxane polymers include resistance to ultraviolet light, extremely low glass transition temperature, good thermal and oxidative stability, good permeability to many gases, very low surface energy, low index of refraction, good hydrophobicity, and good dielectric properties. They also have very good biocompatibility and are of great interest as biomaterials which can be used in the body in the presence of blood. Polydiorganosiloxane elastomers have been widely used because of these many excellent properties. But, their limited tear resistance and poor resistance to low polarity solvents have made them unsuitable in many other applications.
In recent years, free radically curable and moisture curable liquid polydiorganosiloxane compositions have been disclosed that cure rapidly and completely under exposure to radiation, moderately elevated temperatures, or moisture with excellent properties. Thus, subsequent manufacturing or repair steps wherein these compositions are used are often delayed until some degree of curing occurs. Also, thick constructions cannot be made without temporary support until curing is accomplished, and irregularly shaped surfaces can be difficult to coat adequately. Therefore, there is still a need for polydiorganosiloxane compositions with green strength, i.e., strength in the uncured state, and controlled flow properties.
Polydiorganosiloxane segmented copolymers is polydiorganosiloxane urea containing components which may contain blocks other than polydiorganosiloxane or urea. These copolymers have some potential process economy advantages because their synthesis reaction is more rapid than those previously mentioned, requires no catalyst, and produces no by-products.
Traditionally, polydiorganosiloxane pressure-sensitive adhesives have been made in solution. Conventional solvent based polydiorganosiloxane pressure-sensitive adhesives are generally blends of high molecular weight silanol functional polydiorganosiloxanes, i.e., polydiorganosiloxane gums, and copolymeric silanol functional silicate resin, i.e., MQ resins, which comprise R3SiO1/2 units and SiO4/2 units. Improvements in such pressure-sensitive adhesive properties are achieved when the copolymeric polydiorganosiloxane resin and polydiorganosiloxane are intercondensed, providing intra- and inter-condensation within the adhesive. This condensation step requires 1) the addition of a catalyst, 2) reacting the copolymeric polydiorganosiloxane resin and polydiorganosiloxane in solution, and 3) allowing the reaction to take place over a period of time at elevated temperature.
Solution polymerized polydiorganosiloxane urea elastomers that do not require a cure step have been described. However, because these compositions are made in solvent, they can have costly handling procedures.
Continuous melt polymerization processes are advantageous and have been used to make compositions such as polyurethane elastomers and acrylate pressure sensitive adhesives. A continuous melt polymerization process for producing polyetherimides, which can contain polydiorganosiloxane segments, has also been described. Recently, polyurethane resins have been described which use polydiorganosiloxane urea segments to prevent blocking of film formed from the resin. However, levels of reactive polydiorganosiloxane in the compositions were small, for example, less than 15 weight percent, and potential incomplete incorporation of the polydiorganosiloxane into the backbone was not detrimental since easy release was the intent. Unincorporated polydiorganosiloxane oil can, however, act as a plasticizing agent in elastomers to reduce tensile strength or detackify and reduce shear properties of pressure-sensitive adhesives. This unincorporated oil can also bloom to the surface of an elastomer or adhesive and contaminate other surfaces with which it is in contact.
Mixtures of polymeric components have also been used in various applications. Enhanced peel adhesion performance has been seen when acrylic pressure-sensitive adhesives have been melt mixed with thermoplastic elastomers and subsequently extrusion coated onto various substrates. Polydiorganosiloxane urea has been mixed in solvent with dielectric polymers to form dielectric layers for the imaging sheets of an electrostatic printing process that releases more easily from later applied toners. However, good images result only when the polydiorganosiloxane urea contains a non-polydiorganosiloxane hard segment of at least 50 weight percent.